The Scottish borders fens : controls on vegetation development and composition.

The Scottish Borders fens represent an important group of British fen sites. Despite their
small size many of these fens support various rare plant communities and nationally and
locally rare plant species as well as a wide range of species characteristic of wetland habitats.
This study has demonstrated the importance of management history, site chemical conditions
and site size and morphology in the vegetation development of the Scottish Borders fens.
2. Sixty-eight sites were included in a general survey. The vegetation at these sites was
systematically recorded. Site features (vegetation rafts, springs, drains), surrounding land-use,
gross peat stratigraphy and measurements of pH and electrical conductivity of the fen water
were also recorded. A subset of contrasting sites was selected for detailed investigations into
site chemical conditions and peat fertility, peat stratigraphy and the development of vegetation
rafts.
The quadrat data were analysed using a range of multivariate classification procedures. One
classification was selected as the basis for the description of Scottish Borders fen plant
communities. Twenty-four plant communities and variants are described representing rich-fen,
poor-fen, bog, tall herb fen, fen meadow and swamp habitat types. The Scottish Borders fen
plant communities were compared to existing comprehensive classifications of British fen
vegetation.
4. The impact of marl and peat extraction at each site was investigated using documentary,
visual and stratigraphic evidence for disturbance at each site. The status of each site was
determined (cutover, partly cutover, un-disturbed) and this was related to the development of
the present vegetation. Most of the Scottish Borders fens have been cut for peat or marl to
some extent. However at some sites the peat stratigraphic sequences represent un-modified
peat development since the late glacial. The efficiency of drains, strength of springs and basin
morphology are important factors determining the development of vegetation over former peat
and marl cuttings.
5. Site chemical conditions show much variation. The intensity of the surrounding land-use is
positively correlated with peat fertility at the edges and water inflows of sites. The main axes
of floristic variation correspond to the variation in base-richness of the fen water and fertility.
There was no simple relationship between chemical variables and the occurrence of different
plant communities.
6. The vegetation has developed as a quaking raft over fluid peat at many sites. Two Sphagnum
dominated plant communities are confined to vegetation rafts. Their occurrence is determined
by the inundation of the vegetation surface with base-rich water, the depth of the fluid peat
beneath the raft and the degree of isolation of the vegetation surface from telluric water input.
Where the vegetation has developed as a raft over the entire site the thickest rafts are found in
the central areas. The mechanisms of central raft thickening are thought to be influenced by
differential fluctuation of the water table relative to the vegetation surface across sites.